As a result of their exceptional mechanical-technological properties, polymers are widely used in the mechanical engineering industry, in electrical engineering, in the building trade, in the textile industry, in the paints industry and for objects of everyday use. Production takes place either by interface condensation or melt polycondensation by direct polycondensation from dicarboxylic acids and dialcohols or diphenols or by transesterification of the corresponding acid esters. When melt polycondensation is used to produce polycarbonates and polyphosphonates, aromatic dihydroxy compounds, e.g. bis(4-hydroxyphenyl)alkanes, especially bisphenol A, are transesterified with diphenyl carbonate or diarylalkyl phosphonates in the presence of catalysts with the separation of phenol, oligomerised and then subjected to polycondensation. The polycondensation takes place in several reaction steps under increasing vacuum, for example, beginning with a gentle vacuum of |800| mbar, a vacuum of <|100| bar is set for the pre-polycondensation, and a vacuum of <|1| mbar is set in the end step for the polycondensation at a temperature of 220 to 350° C.
When the vacua are produced by means of positive-displacement vacuum pumps followed or preceded by surface condensers to separate the condensable components contained in the vapours produced during the polycondensation, such as phenols, multivalent alcohols, monomer or oligomers, the condensable components are deposited at suitably low condensation temperatures in the surface condensers and/or in the pump and piping system so that interruptions of operation occur. To avoid this disadvantage, coolers with rotating scrapers have been proposed to clean the cooling surfaces. A disadvantage however is that in the event of a leak from the shaft passage which is under vacuum, there is a major risk for operation and product quality. It is also known (SRI Report No. 50B [1982] Polycarbonates, FIG. 5.1) to produce the vacuum by means of two water-vapour operated steam-jet vacuum pumps arranged one after the other in the final stage of the polycondensation whilst retaining the surface condenser. In this case, the waste water is contaminated, for example, by the accumulating phenols, dialcohols and oligomers. In addition, the oligomers are deposited in the steam-jet vacuum pumps.
U.S. Pat. No. 3,468,849 and DE-A-2227261 describe methods for producing polyethylene terephthalate (PET) wherein the vacuum is produced in the final stage of the polycondensation by means of a steam-jet vacuum pump driven by ethylene glycol (EG) vapour at a pressure of approx. |2| bar. EG is liquid at room temperature and boils at a pressure of |2| bar and a temperature of 222° C. whereas the monomeric initial products of the production of polycarbonates, polysulfones, polyether ketones, polyarylates and polyphosphonates are solid at room temperature and have high boiling points of >300° C. at atmospheric pressure; in this case, partly undesirable decomposition and side reactions occur. In the polycondensation of PET, the EG which is both the working medium of the steam-jet vacuum pump and the monomer is separated and accumulates under normal conditions in liquid form.
During the polycondensation of polycarbonates from diphenyl carbonate, of polyarylates from dicarboxylic acids, of polyphosphonates from bisphenols and/or other phenyl esters, various phenols are formed from the monomers or the separation products from the transesterification reaction in the production of polyesters, which are toxic and become solid at a temperature below 41° C. These properties impair the safety of the operation and staff and cause corrosion effects.
The subject matter of DE-A-4440741 is a method for producing a vacuum and separating the condensable components from the vapours of melt-phase polycondensation in the production of polycarbonate. In this method the suction side of a polycondensation stage is connected to at least one to two steam-jet vacuum pumps with a downstream injection condenser. The working vapour is diphenyl carbonate vapour at a pressure of |0.3| to |1.5| bar and the spray liquid is liquid diphenyl carbonate. This method makes it possible to advantageously produce vacuum in one or more reaction stages during the production of polycarbonate by melt-phase polycondensation. Operating interruptions caused by deposition of oligomers or by saturation of surface condensers are almost eliminated. Emissions to be disposed of are reduced to a minimum, in particular no waste water contaminated with phenol, oligomers and monomers accumulates.